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What is focus shift?

Focus shift is the phenomenon which occurs when you adjust focus in a different light to the light that the actual image is taken in – the result is a shift in the focus plane on the subject. In the past for UV-imaging, the focusing was done in visible light (without a UV-pass filter on the lens) but the subject was shot in UV light (with the UV-pass filter on).

This phenomenon is due to the optical characteristics of the lenses which are not corrected for UV light, just like what happens with chromatic aberration. So though some of these lenses pass a good amount of UV light through to the sensor, the UV light is focused either in front of or behind the intended focus point of the subject on the sensor. Hence the subject will appear to be out of focus at the original focus point.

2) Mark on the lens the known focus shift expected and make adjustments of the lens to compensate for this after focusing in visible light i.e. manually correcting for the focus shift of the lens.

Liveview focusing bypasses this problem as what you see is what you get (WYSIWYG), as focusing can now be done in UV-light which the modified camera is sensitive to (and which our eyes can’t see). And as the focusing is done in the light the subject is being shot in, it is irrelevant whether there is focus shift in the lens used.

But if imaging both UV and visible light at the same time, you will require a lens with minimal or no focus shift. Hence it is still good to have a lens with such capabilities.